1. Field of the Invention
The present invention relates to a fuel cell system and a method for controlling the stoppage of the fuel cell system.
2. Description of the Related Art
Known fuel cells mounted in fuel cell vehicles include a fuel cell stack including flat unit fuel cells (hereinafter, referred to as “unit cells”) each configured such that a polymer electrolyte membrane is sandwiched between an anode and a cathode to form a membrane electrode assembly (MEA) and the MEA is disposed between two separators. In each fuel cell, hydrogen is supplied as an anode gas (fuel) to the anode and air is supplied as a cathode gas (oxidizer) to the cathode. Hydrogen ions generated by catalytic reaction at the anode permeate through the electrolyte membrane to the cathode. The hydrogen ions electrochemically react with oxygen in the air at the cathode (O2+4H++4e−→2H2O), thus generating electricity.
In the above-described fuel cell, during stopping of a vehicle, particularly, while an idle stop is performed (i.e., the fuel cell is temporarily stopped), a discharge process for consuming hydrogen remaining in the fuel cell may be performed in order to prevent hydrogen from being diffused (crossover) and being then discharged to a diluter on the next start-up.
In some cases, however, it is better that the discharge process should not be performed depending on the internal state of the fuel cell. For example, the anode gas may not be delivered to the entire surface of the fuel cell (unit cell) because of, for example, a flooding phenomenon, in which water produced by electricity generation locally accumulates in the fuel cell to interfere with the supply of the anode gas, or a blockage in a flow passage for the anode gas. If the discharge process is performed in such a state, the anode gas lacks relative to a requested electric current to be generated (a lack of balance in stoichiometry which will be described later). Disadvantageously, this leads to degradation of the polymer electrolyte membrane of the fuel cell.
Japanese Unexamined Patent Application Publication No. 2007-258117 discloses a system in which a voltage (cell voltage) parameter threshold value is calculated on the basis of information indicating a load just before an idle stop and the parameter threshold value is compared to a measured voltage parameter to determine whether to permit the idle stop.